The measurement of mean and fluctuating wall shear-stress in laminar, transitional, and turbulent boundary layers and channel flows has applications both in industry and the scientific community. Time-resolved, fluctuating shear-stress data can also provide physical insight into complex flow phenomena, including turbulent viscous drag, transition to turbulence, flow separation, and shock-wave/boundary layer interactions. For example, the accurate measurement of skin friction is important to the aircraft industry.
Unfortunately, macro-scale measurement technology is insufficient to meet the demands of directly obtaining accurate mean and fluctuating wall shear stress data. More specifically, the accurate, direct measurement of fluctuating wall shear stress has not been realized via conventional measurement technology.
Micromachining technology provides the opportunity to synthesize transducers possessing superior performance compared to mainstream mechanical fabrication techniques. Specifically, the small physical size and corresponding reduced mass of micro-sensors offers the potential to vastly improve both the temporal and spatial measurement bandwidth.
Realizing the potential advantages of miniaturization scaling, the MEMS community has developed both thermal, floating element, and optical shear-stress sensors. Thermal sensors are generally robust and simpler to fabricate. However, they are based on a heat transfer analogy and absolute calibration for quantitative measurements is difficult. Optical MEMS (MOEMS)-based laser-Doppler anemometers that measure velocity gradients in the viscous sublayer are also known, but the ability to generate a sufficiently small measurement volume in a high-Reynolds number sublayer is challenging.
Floating-element structures provide a good opportunity to obtain direct quantitative, time-resolved measurements in a controlled wind tunnel environment. Several transduction techniques are known for measurement of the shear-stress induced deflection of floating elements, including capacitive, piezoresistive, and differential optical shutter techniques. However, such techniques have limitations, including thermal management issues, lacking the ability to be flush-mountable with no wire bonds that generate flow disturbances, and being subject to electromagnetic interference and pressure fluctuations.